o. varnavski, g. menkir, t. goodson iii and p. l. burn- ultrafast polarized fluorescence dynamics in...

8/3/2019 O. Varnavski, G. Menkir, T. Goodson III and P. L. Burn- Ultrafast polarized fluorescence dynamics in an organic dend…

http://slidepdf.com/reader/full/o-varnavski-g-menkir-t-goodson-iii-and-p-l-burn-ultrafast-polarized 1/3

Ultrafast polarized fluorescence dynamics in an organic dendrimer

O. Varnavski, G. Menkir, and T. Goodson IIIa)

Department of Chemistry, Wayne State University, Detroit, Michigan 48202

P. L. Burn Dyson Perrins Laboratory, South Parks Road, Oxford, OX1 3QY United Kingdom

Received 23 December 1999; accepted for publication 28 June 2000

The excited state relaxation processes of a nitrogen-cored distyrylbenzene-stilbene A-DSBdendrimers and the analogous linear model compound bis-MSB were investigated by polarized

fluorescence upconversion spectroscopy. The fluorescence anisotropy FA of A-DSB was found to

decay to a value close to zero in less than 200 fs after excitation. For the model compound bis-MSB

the FA initial value was close to 0.4 and showed a relatively slow decay 82 ps corresponding to

the overall rotational diffusion of the molecule. The results are interpreted in terms of fast transition

dipole reorientation during relaxation of the excited states of the branched molecules. © 2000

American Institute of Physics. S0003-69510002334-2

Recently there has been an increasing amount of atten-

tion devoted to the study of dendrimers, which are highly

branched macromelecular systems.1,2 Their unique branched

architecture affords them many properties, which are distinc-

tively different from those of their linear analogs.3,4 Along

with the synthesis and chemical properties, the photophysics

of these systems is attracting rising interest in connection

with their applications as artificial antenna systems.5,6 How-

ever, the origin of electronic excitations and specific mecha-

nism of intramolecular energy transport in dendrimers is not

well understood.4,6,7 The branching centers such as benzene

with a meta-position linkage5,8 or nitrogen3 can cause a

disruption of the -electron conjugation of linear building

blocks. In this case the optical excitation creates the electron-

hole pairs localized on small segments.4

However, these localized electron-hole pairs can contrib-ute to either collective optical excitations excitons4 or to

the incoherent hopping6 due to intersegment interactions.

Each step of the incoherent energy migration in a branched

system should lead to the reorientation of the transition di-

pole moment and depolarization of the emission. The hop-

ping time hopping can be related to the mutual orientation of

the dipoles in dendrons and the depolarization time depolar

in a simple case of a planar system as9

dopolar

hopping

41cos2

. 1

A more general description of fast depolarization in a

symmetrical branched molecular system is the relaxation of a

superposition of degenerate states with different orientations

of transition dipoles. An analysis of the fluorescence anisot-

ropy associated with two-fold and three-fold degenerate

states with different orientations of the transition dipole has

been given by Wynne and Hochstrasser.10 Other manifesta-

tions of coherent effects in anisotropy optical experiments

were also theoretically analyzed.11–13 In this letter we report

the time-resolved polarized inherent fluorescence of den-

drimers with 200 fs time resolution.

The structure of the nitrogen-cored distyrylbenzene-

stilbene A-DSB is shown in Fig. 1a. Three distyrylben-

zene chromophores are grouped in a tetrahedral arrangement

around the nitrogen and there is a lone electron pair. The

dendrons are based on metapositioned stilbene units. The

molecules of zero G0 and the second G2 generations are

shown in Figs. 1a and 1b. The linear absorption spectrum

is shown in the inset of Fig. 2. It shows two prominent fea-

tures with maxima at 410 and 320 nm. The first feature cor-

responds to the absorption of DSB chromophores and the

second one can be assigned to the stilbene units. The struc-

ture of a nonlinear optical dendrimer also used in this study

is depicted in Fig. 1c. Synthesis and spectroscopic featuresof all the structures have been described elsewhere.8,14 The

p-biso-methylstyryl-benzene bis-MSB chromophore was

used as a model compound for investigations of intermolecu-

lar interactions in A-DSB. Solutions in chloroform were used

in this study.

Femtosecond upconversion spectroscopy was employed

to resolve temporally the polarized fluorescence. Optical ar-

rangement for the upconversion experiments has been de-

scribed previously.14–16 The sample was excited with laser

pulses delivered by a frequency doubled output of the Ti:sap-

phire laser Tsunami, Spectra Physics. The laser with an

average pulse width of 100 fs tuned at 790–860 nm and a

repetition rate of 82 MHz. The polarization of the excitation

aElectronic mail: [email protected]

FIG. 1. The structure of A-DSB dendrimers for zero G0 a and second G2

b generations. c The structure of CZD4NS2.

APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 8 21 AUGUST 2000

11200003-6951/2000/77(8)/1120/3/$17.00 © 2000 American Institute of PhysicsDownloaded 01 Oct 2008 to 138.251.42.50. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp



beam was controlled using Berek compensator. The time

resolution was determined by the pulse width of the laser andgroup velocity dispersion within optical elements of the sys-

tem. The full width at half maximum of the cross-correlation

function at 790/395 nm was estimated to be 190 fs. The

rotating sample cell and a holder 1 mm thickness were used

to avoid the thermal and photochemical accumulative ef-

fects. Excitation average power was kept at the level of 0.5

mW.

The fluorescence isotropic decay of A-DSB on the pico-

second time scale was found to be nonexponential and

strongly dependent on the emission wavelength Fig. 2. The

effect of fluorescence rise time is clearly detected in the

‘‘red’’ regions of the fluorescence spectrum. This fluores-

cence time behavior can be assigned either to intramolecularinteractions in the dendrimer excimer formation, energy

transfer17 or to local relaxation processes in the chro-

mophore conformational changes, solvent rearrangement.15

The long-scale decay pattern not shown was found to be

almost mono-exponential with the decay time of about 1 ns.

The fluorescence dynamics detected at 515 nm were found to

be the same for both generations when excited at 395 nm.

To gain a further insight into energy transfer energy

migration processes in A-DSB dendrimer molecules we in-

vestigated the fluorescence anisotropy dynamics in G0 and

G2. The result for G2 is shown in Fig. 3. Raw fluorescence

anisotropy R( t ) was calculated from the decay curves for the

intensities of fluorescence polarized parallel I par( t ) and per-pendicularly I per( t ) to the polarization of the excitation light

according to the equation

R t I par t G* I per t

I par t 2G* I per t . 2

The factor G accounts for the difference in sensitivities

for the detection of emission in the perpendicular and paral-

lelly polarized configurations. We obtained this factor from

our test measurements of fast rotational diffusion of perylene

in solution.18 It is clearly seen in Fig. 3a that the fluores-

cence anisotropy decays to a small residual value of about

0.05 within the instrument response function duration 200fs and remains almost unchanged on the picosecond time

scale Fig. 3b. The initial spike at about zero delay is

relatively small and comparable in duration with the time

resolution of our system to be reliably analyzed quantita-

tively. Qualitatively, it reflects the fact that the initial decay

of R(t ) is short compared to time resolution of our upcon-

version setup 200 fs. The fluorescence anisotropy dy-

namics of the dendrimer CZD4NSC2 and of bis-MSB are

also shown in Fig. 3 for a comparison. It is clearly seen that

initial values of R( t ) for last two cases are in the range

0.3–0.4, close to the limiting value of 0.4. It should be noted

that in our previous work no signs of interchromophore in-

teraction was found for CZD4NSC2.15 The R(t ) decay times

of 600 ps CZD4NSC2 and 82 ps bis-MCB are most prob-

ably connected with overall rotational diffusion of these mol-

ecules. For G0, G2 there were no visible signs of molecular

overall rotation as the fluorescence anisotropy dropped to a

very small residual value within the time interval of 200 fs,

which is much shorter than a reasonable time of rotational

diffusion.It is clearly seen from Fig. 3a that the fluorescence

FIG. 2. Fluorescence isotropic decay of G0 in chloroform for different

emission wavelengths, when excited at 395 nm. Inset: absorption spectra of

G0 and G2 excitation wavelengths are indicated by arrows.

FIG. 3. Time-resolved fluorescence anisotropy of A-DSB, CZD4NS2 den-drimers, and bis-MSB model compound. Excitation wavelength—395 nm.

a Short time scale. Comparison of G2, and bis-MSB. Emission wave-

lengths are 480 and 450 nm, respectively. Fluorescence anisotropy of G0 is

also shown for comparison. b Medium time scale. Comparison of G2 and

bis-MSB. c Long time scale. Comparison of G2 and CZD4NS2. Emission

wavelengths are 480 and 630 nm, respectively.

1121Appl. Phys. Lett., Vol. 77, No. 8, 21 August 2000 Varnavski et al.

Downloaded 01 Oct 2008 to 138.251.42.50. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp



anisotropy dynamics are very similar for G0 and G2 indicat-

ing that the fast fluorescence depolarization process is asso-

ciated with the dendrimer core and not sensitive to presence

of the stilbene chromophore. The dendrimer core G0 is a

star-like molecule consisting of three DSB segments bonded

to a central nitrogen atom. The interaction between branches

could lead either to the formation of coherent excitonic

states10 or hopping-type19 relaxation of excitations localized

on one DSB branch. Both relaxation mechanisms betweenexcitonic states10,13 and incoherent hopping19 may lead to

fast fluorescence depolarization due to the star-like organiza-

tion of the branches. Indeed, for the linear polymeric system

polyS-119 we found no fast depolarization with interacting

side chain chromophores.16

From further experimental investigations we did not ob-

serve any detectable change in the fluorescence anisotropy

decay pattern with excitation wavelength variation in the

range 380–410 nm see arrows in Fig. 2. This clearly proves

that the fast depolarization is not associated with an energy

transfer from the stilbene dendrons to the core. Also the sug-

gestion of simultaneous excitation of two different energy

levels with different polarizations20 in G0 resulting in an

anisotropy value of zero can be ruled out on the basis of

these experiments.

It may be suggested as well that the fluorescence anisot-

ropy of G0 is nearly zero because of an irreversible relax-

ation to a state with the transition dipole oriented at about

55° magic angle with respect to the initially excited one.

This case is physically similar to the process of energy trans-

fer between branches suggested in our manuscript. However,

this simple two-state model is not reasonably applicable to

our particular symmetrical geometry. For example in the

A-DSB system a more realistic angle of 60° 120° and for

the case of one irreversible step the fluorescence anisotropy

FA should be negative 0.05. However, our results show

a positive value close to what is expected for multistep

equilibration process in a symmetrical molecule with three

branches.

In conclusion, fast fluorescence depolarization within

200 fs was found for the A-DSB dendrimer. For the model

system bis-MSB representing the linear building block of the

dendrimer a FA decay time of 82 ps was obtained which

agrees with the rotational diffusion of bis-MSB. These re-

sults demonstrate the ultrafast transition dipole reorientation

due to intersegment interactions in A-DSB dendrimer sug-

gesting energy transfer rate of 1.71012 s1 in case of

hopping mechanism. The anisotropy results of CZD4NSC2

support our previous conclusions concerning the lack of de-

tectable excited state interchromophore interactions in this

particular dendrimer. Time-dependent fluorescence anisot-ropy measurement showed to be a powerful method for the

evaluation of the intramolecular interactions in the A-DSB

organic dendrimer.

1 D. A. Tomalia, A. N. Naylor, and W. A. Goddard III, Angew. Chem. Int.

Ed. Engl. 29, 138 1990.2 J. M. J. Frechet, Science 263, 1710 1994.3 D. A. Tomalia and P. R. Dvornic in Polymeric Materials Encyclopedia,

edited by J. C. Salomone CRC Press, Boca Raton, FL., 1996, Vol. 3, p.

1814.4 S. Tretiak, V. Chernyak, and S. Mukamel, J. Chem. Phys. 110, 8161

1999.5 R. Kopelman, M. Shortreed, Z.-Y. Shi, W. Tang, Z. Xu, J. Moore, A.

Bar-Haim, and J. Klafter, Phys. Rev. Lett. 78, 1239 1997.6 A. Bar-Haim, J. Klafter, and R. Kopelman, J. Am. Chem. Soc. 119, 6197

1997.7 S. Tretiak, V. Chernyak, and S. Mukamel, J. Phys. Chem. B 102, 3310

1998.8 J. N. G. Pillow, M. Halim, J. M. Lupton, P. Burn, and I. D. W. Samuel,

Macromolecules 32, 5985 1999.9 R. Jimenez, S. N. Dikshit, S. E. Dradforth, and G. R. Fleming, J. Phys.

Chem. B 100, 6825 1996.10 R. K. Wynne and R. M. Hochstrasser, Chem. Phys. 171, 179 1993.11 R. Knox and D. Gulen, Photochem. Photobiol. 57, 40 1993.12 A. Matro and J. A. Cina, J. Phys. Chem. 99, 2568 1995.13 R. Kumble and R. M. Hochstrasser, J. Chem. Phys. 109, 855 1998.14 O. Varnavski, L. Liu, J. Takacs, and T. Goodson III, J. Phys. Chem. 104,

179 2000.15 O. Varnavski, A. Leanov, L. Liu, J. Takacs, and T. Goodson III, Phys.

Rev. B 61, 12732 2000.16 O. Varnavski and T. Goodson III, Chem. Phys. Lett. 320, 688 2000.17 Y. Karni, S. Jordens, G. DeBelder, G. Schweitzer, J. Hofkens, T. Gensch,

M. Maus, F. C. DeSchryver, A. Herrmann, and K. Mullen, Chem. Phys.

Lett. 310, 73 1999.18 S. N. Goldie and G. J. Blanchard, J. Phys. Chem. A 103, 999 1999.19 L. Larettini, G. DeBelder, G. Schweitzer, M. Van der Auweraer, and F. C.

DeSchryver, Chem. Phys. Lett. 295, 11 1998.20 M. van Gurp, T. van Heijnsbergen, G. van Ginkel, and Y. K. Levine, J.

Chem. Phys. 90, 4103 1988.

1122 Appl. Phys. Lett., Vol. 77, No. 8, 21 August 2000 Varnavski et al.

Downloaded 01 Oct 2008 to 138.251.42.50. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

o. varnavski, g. menkir, t. goodson iii and p. l. burn- ultrafast polarized fluorescence dynamics in...

Documents